Steel Diagrid Research Articles

Quantification of seismic performance factors of steel diagrid system

SummaryThis study is carried out to evaluate the response modification, overstrength, and deflection amplification factors for steel diagrid system. To quantify these factors, the rational procedure introduced in Federal Emergency Management Agency (FEMA) P695, which is based on low probability of structural collapse and encompasses nonlinear static and dynamic analyses, is used. To this end, a series of diagrid structures with various slopes of external braces are designed. Nonlinear static analyses are performed to obtain system overstrength and period‐based ductility. Incremental dynamic analyses are then performed to assess collapse margin ratio (CMR) of the archetypes. The effects of spectral shape of different ground motions are considered in CMR values to obtain an adjusted CMR (ACMR), for each archetype. The values of calculated ACMRs are compared with the acceptable values of FEMA P695 to assess reliability of presumed seismic performance factors of diagrid systems. The results show that values of calculated ACMRs for steel diagrid systems designed using the response modification factor equal to 4.50 are acceptable. It is also observed that as the slope of the peripheral diagonal elements increases, the overstrength and the collapse margin ratios decrease, and the ductility of system increases.

Performance-Based Seismic Design and Evaluation of Fused Steel Diagrid Frame

A fused steel diagrid frame (FSDF) is an innovative earthquake resilient structural system. FSDF combines the steel diagrid structural system with shear links (SLs) and moment connections (MCs) to dissipate earthquake energy. SLs are placed between diamond-shaped grid units and serve as the primary seismic force-resisting system (SFRS). MCs provide the additional resistance as the secondary SFRS. To facilitate the design of the proposed FSDF system, a performance-based equivalent energy design procedure (EEDP) is utilized to design a prototype building. EEDP allows designers to select multiple performance objectives under different earthquake hazard intensities. A detailed finite element model is developed to simulate the nonlinear dynamic response of the structure under earthquake shaking intensities. The results of nonlinear dynamic analyses show that the FSDF has superior seismic performance and can be efficiently designed using EEDP. Lastly, detailed collapse risk assessment of the prototype building utilizing the FSDF is conducted using the Federal Emergency Management Agency FEMA P695 (2009) methodology. The result shows that the EEDP-designed FSDF has superior margin against collapse.

Nonlinear Behavior and Design of Mid- to High-Rise Diagrid Structures in Seismic Regions

The aesthetics and structural advantages of the diagrid structural system have made it an attractive choice for many buildings across the world, including several notable high-rise building structures built in recent years. This paper presents an investigation of nonlinear behavior and design of mid- to high-rise steel diagrid structures. Weight, story drift, fundamental period, lateral stiffness, and sequence of plastic hinge formation in steel diagrids are studied and compared with corresponding moment-resisting frames and concentrically braced frames. To improve the nonlinear behavior and increase the collapse load capacity of diagrid structures in high seismic regions, practical design guidelines are proposed by using virtual work/energy diagrams and by performing nonlinear static analysis. So far, the diagrid system has been used mostly in the design of tall buildings in the range of 20 to 100 stories. A conclusion of this research is that the diagrid system can also be an efficient and economical structural system for mid-rise buildings in the 8- to 15-story range.

Seismic performance factors for low‐ to mid‐rise steel diagrid structural systems

SummaryDiagrids are known as an esthetically pleasing and structurally efficient system. The current design codes and provisions, however, provide no specific guidelines for their design under extreme events such as earthquakes. This paper presents a comprehensive investigation of the performance of steel diagrid structures to evaluate their key seismic performance factors. Nonlinear static, time‐history dynamic, and incremental dynamic analyses are used to assess diagrid performance and collapse mechanisms in a high seismic region. Seismic performance factors including response modification factor, ductility factor, overstrength factor, and deflection amplification factor are quantified using 4 different methodologies. Four archetype groups of diagrid buildings ranging in height from 4 to 30 stories have been investigated. An R factor in the range of 4 to 5 is recommended for steel diagrid frames in the range of 8 to 30 stories unless supplementary analyses are conducted to find the optimal diagonal angle. For low‐rise steel diagrids (under 8 stories), an R factor in the range of 3.5 to 4 is recommended. Further, an overstrength and ductility of 2.5 and 2 are recommended. This paper lays the groundwork for including steel diagrids in design provisions.

Seismic Performance Evaluation of Steel Diagrid Buildings

In this study the seismic performances of axi-symmetric steel building structures with circular plan shape were evaluated based on the ATC-63 approach. For analysis models, thirty-three-story vertically convex, concave, and gourd-type axi-symmetric buildings were designed using diagrid structure system, and their seismic performances were compared with those of the cylinder type regular steel structure. Seismic fragility analyses were carried out using twenty-two pairs of earthquake records to obtain the probability of failure for a given earthquake intensity. The validity of the response modification factor used in the seismic design of the model structures was also investigated. Based on the analysis results it was concluded that the response modification factor of 3.0 used in the design of the model structures is acceptable for the ATC-63 methodology. It was also observed that the seismic safety margin for a specific level of earthquake decreases as the vertical irregularity of the structure increases.

Quantification of the seismic performance factors for steel diagrid structures

There are a number of studies on seismic performance of a diagrid structure indicating that it is an effective choice for constructing tall buildings. This research investigates the seismic behavior of diagrid structures and quantifies its seismic performance factors including the response modification coefficient (R-factor), the over-strength factor (Ω0) and the displacement amplification factor (Cd) based on the FEMA P695 methodology. In that regard, a group of 3-D steel diagrid archetype models with different number of floors and various diagonal angles are designed using different R-factors. Then, in the first step, the over-strength factors of these models are determined by performing nonlinear static analyses. Then, utilizing the incremental dynamic analysis (IDA), the median collapse capacity and collapse margin ratio (CMR) of these models are calculated and their Cd factors are estimated using the computed R-factors. The obtained results indicate that the R-factor for steel diagrid systems depend on the diagonal angles. For diagrids with angles of 45°, 63.4° and 71.5°, amounts of R-factor were determined as 1.5, 2, and 3, respectively. Moreover, the pin and rigid types of end-connection of diagrid perimeter beams was found to have no effects on stiffness of the diagrid models. However, the IDA results indicate that the diagrids with pin-ended beams tolerate larger collapse displacements especially in shorter models. Furthermore, replacing the rigid-ended beams by pin-ended beams improves the seismic performance of diagrids, particularly for models with larger diagonal angles (63.4° and 71.5°). The OpenSees program was used for modeling and numerical analyses.

Comparison of the performance of concrete-filled steel tubular and hollow steel diagrid buildings

In the recent construction scenario, diagrid structures are becoming a popular high-rise building structural system. Diagrid structures consist of diagonals in the perimeter and an interior core. The corner and interior vertical columns are not required due to the structural efficiency of diagrid structural systems. Steel and concrete are commonly used material for diagrid. An alternate material for diagrid is concrete-filled steel tube (CFST). CFST incorporates the advantages of both steel and concrete. In CFST, the inward buckling of the steel tube is effectively prevented by the filled concrete. The compressive strength of concrete increases due to the tri-axial state of stress in concrete induced by the steel tube. The longitudinal as well as lateral reinforcement to the concrete core is also provided by the steel tube. This paper compares the performance of CFST and steel diagrid buildings using linear static analysis. For this purpose, a 12 storey and 36 storey building are analysed using finite element method and CFST diagrid building is found to perform better.

Diagrid Structural System for High-Rise Buildings: Applications of a Simple Stiffness-based Optimized Design

The ingenuity of structural engineers in the field of tall and super-tall buildings has led to some of the most remarkable inventions. During this evolution of structural engineering concepts in the last 100 years, the technical challenges that engineers encountered were extraordinary and the advances were unprecedented. However, as the accomplishments of structural engineers are progressing, the desire for taller and safer structures is also increasing. The diagrid structural system is part of this evolving process as it develops a new paradigm for tall building design combining engineering efficiency and new architectural expression. The first appearances of this type of tall buildings have already been constructed and the interest of both engineering and architectural communities is growing mainly due to the many advantages compared to other structural systems. This paper presents a simple approach on optimizing member sizes for the diagonals of steel diagrid tall buildings. The optimizing method is based on minimizing the volume of the diagonal elements of a diagrid structure. The constraints are coming from the stiffness-based design, limiting the tip deflection of the building to widely accepted regulative limits. In addition, the current paper attempts to open the discussion on the important topic of optimization and robustness for tall buildings and also studies the future of the diagrid structural system.

Analytical Study on Effect of Geometry of Tall Buildings on Diagrid Structural Systems Subjected to Lateral Loads

Structural design of high rise buildings is governed by lateral loads due to wind or earthquake. As the height of building increases, the lateral load resisting system becomes more important than the structural system that resists the gravitational loads. Recently, diagrid structural system are widely used for tall buildings due to its structural efficiency and flexibility in architectural planning. Diagrid structural system is made around the perimeter of building in the form of a triangulated truss system by intersecting the diagonal and horizontal members. Diagonal members in diagrid structural systems can carry gravity loads as well as lateral loads. Lateral loads are resisted by axial action of the diagonals compared to bending of vertical columns in framed tube structure. The structural efficiency of diagrid system also helps in avoiding interior and corner columns, thereby allowing significant flexibility with the floor plan. In this paper, effect of lateral loads on steel diagrid buildings are studied. Square and rectangular buildings of same plan area with diagrid structural system is considered for the study. Diagrid modules extending upto 2,4,6,8 and 12 storeys are evaluated. Static analysis for the gravity loads, wind and earthquake and response spectrum analysis are carried out for these different combinations of plan shape and diagrid modules and performance of all these diagrid models i.e., storey displacement, storey drift and modal time period are evaluated and compared in this study.

A Review of Diagrid Systems for Lateral Load Resistance of Structures

Diagrid structures are exterior structures, consisting of diagonal struts and ties in the periphery and an interior core. These diagonal members carry gravity load and lateral load by the axial action of the member. Due to the structural efficiency of diagrids, interior and corner columns can be avoided thereby providing flexibility in the floor plan. The diagrid structures are emerging as popular structural system in many developed countries of the world, but in India it is yet to gain importance. This paper presents a review on the literature of diagrid structures. Studies conducted on diagrid structures to determine the diagrid angle for the efficient design is presented. The stiffness-based methodology adopted for determining preliminary member sizes of steel diagrid structures and the effect of shear lag on high rise buildings with diagrid and its comparison with framed tube structures are discussed. The distribution of the load resisted by interior frame and diagrids is outlined. The parameters used in the comparison of analysis are time period, top storey displacement, inter-storey drift and storey shear. This paper also reviews the studies on the comparison of diagrids with regular configuration and diagrids with varying angles. The analysis and comparison of diagrid and conventional structural system on the basis of consumption of steel, structural weight and displacement are also highlighted. This review covers diagrids with all the materials in practice namely concrete, steel and concrete-filled steel tube(CFST).

Evaluation of seismic performance factors for steel DIAGRID structural system design

This article presents a proposed analytical methodology to determine seismic force-resisting system R-values for steel diagrid framed systems. As current model building codes do not explicitly address the seismic design performance factors for this new and emerging structural system, the purpose of this study is to provide a sound and reliable basis for defining such seismic design parameters. An approach and methodology for the reliable determination of seismic performance factors for use in the design of steel diagrid framed structural systems is proposed. The recommended methodology is based on current state-of-the-art and state-of-the practice methods including structural nonlinear dynamic analysis techniques, testing data requirements, building code design procedures and earthquake ground motion characterization. In determining appropriate seismic performance factors (R, ΩO, Cd) for new archetypical building structural systems, the methodology defines acceptably low values of probability against collapse under maximum considered earthquake ground shaking.

COMPARISON OF SYMMETRIC AND ASYMMETRIC STEEL DIAGRID STRUCTURES BY NON-LINEAR STATIC ANALYSIS

Diagonalized grid structures – “diagrids” - have emerged as one of the most innovative and adaptable approaches to structuring buildings in this millennium. Diagrid is a particular form of space truss, it consists of perimeter grid made up of a series of triangulated truss system. Diagrid is formed by intersecting the diagonal and horizontal components. Construction of multi‐storey building is rapidly increasing throughout the world. Advance in construction technology, materials, structural systems, various analysis and design software have facilitated the growth of various kinds of buildings. Diagrid buildings are emerging as structurally efficient as well as architecturally and aesthetically significant assemblies for tall buildings. Recently these diagrid structural systems have been widely used for tall buildings due to the structural efficiency and aesthetic potential provided by the unique geometric configuration of the system. This paper presents a 12 storey steel diagrid structure which is 36m in height. Symmetric and asymmetric structural configurations of diagrid structures were modelled and analyzed using SAP 2000 by considering Dead load, Live load and Seismic Loads (IS 1893-Part-1, 2002). Then FEMA 356 hinges (auto hinges) are assigned to the same structure and Nonlinear Static (Pushover) analysis is carried out by using seismic load as the pushover load case to find out the performance points that is Immediate Occupancy, Life Safety, and Collapse Prevention of diagrid elements using static pushover curve. At the same time spectral displacement demand & spectral displacement capacity as well as spectral acceleration demand and spectral acceleration capacity is compared to know the adequacy of the design by using ATC capacity spectrum method.

Potentials of elastic seismic design of twisted high-rise steel diagrid frames

This paper is to investigate the potentials of the elastic seismic design of twisted high-rise steel diagrid frame buildings in the strong wind and moderate/low seismicity regions. First, the prototypes of high-rise steel diagrid frames with architectural plans that have a twist angle of 0 (regular-shaped), 1, and 2 degrees were designed to resist wind. Then, the effects of the twist angle on the estimated quantities and structural redundancies of the diagrid frames were examined. Second, the seismic performance of the wind-designed prototype buildings under a low seismicity was evaluated. The response spectrum analysis was conducted for the service level earthquake (SLE) having 43-year return period and the maximum considered earthquake (MCE) having 2475-year return period. The evaluation resulted that the twisted high-rise steel diagrid frames resisted the service level earthquake elastically and most of their diagrid members remained elastic even under the maximum considered earthquake.

Advanced high strength steel tube diagrid using TRIZ and nonlinear pushover analysis

The goal of this study is to provide engineers and designers with a good indication of TRIZ methodologies and evaluate new high performance steel diagrid details to resolve the problematic aspects of steel diagrid structures such as the concentration of stress at the ends of the tube contacted to cap plates. The present steel diagrid is a new trend of structural systems nowadays for free-formed tall buildings, to which high performance steel with tensile stress of 800MPa and a low yielding ratio can be applied.TRIZ is a problem-solving, analysis and forecasting tool derived from the study of patterns of invention in the global patent literature. This study confirms that the TRIZ process can be a strong idea generation tool for developing structural details to improve the behaviors of existing structures. Finally, the effectiveness of the new structural diagrid detail information is presented in this study using actual nonlinear static pushover analysis.

Aldar's new headquarters at Al Raha Beach, Abu Dhabi

Property developer Aldar's new headquarters building in Abu Dhabi sits at the heart of one of the Middle East's biggest single developments – the £13 billion Al Raha Beach project. The iconic, fully glazed structure is completely circular in elevation and curved in all other directions. It also features an external steel diagrid, recognised to be the first such structure in the United Arab Emirates. This paper describes the complex engineering design and construction of the building, which was completed in just 33 months.